The present invention relates to the technology of hydraulic machines. It refers to a hydraulic turbine.
When designing a hydraulic turbine multiple objectives need to be reached simultaneously. Efficiency appears as the predominant one. However, extending the operating range and ensuring a good behaviour of the machine in terms of stability and safety become more and more important. For instance, the customer is now demanding to get a low vibration level in the turbine pit because high intensity vibrations can damage the machine.
These vibrations result from the mechanical response of the structure to hydraulic excitations coming from the interactions between the rotor (the runner) and stator (RSI).
FIG. 1 shows as an example the main parts of a hydraulic turbine of the Francis type. The hydraulic turbine 10 of FIG. 1 comprises a vertical rotor with a runner 12 and a turbine shaft 12a. Runner 12 comprises a plurality of runner blades 16 distributed in a ring around the rotor axis. Rotor 12, 12a is concentrically surrounded by a stator 11, which is arranged between rotor 12, 12a and a surrounding scroll casing 13. Stator 11 is equipped with a plurality of fixed and movable guide vanes 15. Guide vanes 15 direct the stream of water, which is supplied through scroll casing 13 in a circumferential manner, onto runner blades 16 in order to put runner 12 into rotational motion. The water leaves runner 12 in axial direction through a draft tube 14.
The position of the stator 11 relative to the runner 12 (dashed circle in FIG. 1) is shown in detail in FIG. 2. Guide vane 15 of stator 11 extends in vertical direction between an upper stator ring 19 and a lower stator ring 20. Runner blades 16 extend in runner 12 between a runner crown 17 and a runner band 18. Between runner band 18 and the surrounding lower stator ring 20 there is a predetermined clearance C.
Now, the passage of runner blades 16 in front of guide vanes 15, in the so-called vane-less gap VG, produces a local oscillating pressure field especially in case of high head pump turbines (see pressure pulsations 22 in FIG. 3). This dynamic pressure field is mainly due to the pressure variation, on either side of the profiles of guide vanes 15 and runner blades 16, evolving at different frequencies.
Reducing this oscillating pressure field (or pressure fluctuation level) in the vane-less gap VG is a continuous challenge for the designers and manufacturers of hydraulic machines, but few techniques exist to raise this challenge.
Document WO 2008004877 A1 discloses a water turbine of the reaction type, such as a Francis or Kaplan turbine, comprising a rotor and a surrounding housing, with sealing means between the rotor and the housing. In this type of turbine there is a problem in that a large part of the losses is connected with seal leakage. Simple labyrinth seals are often used, but they require large clearances and so have large losses. On this background it is said to be important to provide a water turbine seal system that makes possible much smaller clearances than current labyrinth and other conventional seal designs, so as to obtain an efficiency improvement in high head Francis turbines, inter alia. The proposed sealing means comprises brush seals located at a radial distance from the rotor axis being a major proportion of the maximum radial dimension of the rotor. The disclosure aims at decreasing drastically the fluctuation level with an innovative, easy to set up and low-cost solution.
Document EP 0 565 805 A1 discloses a system for controlling the pulses of hydraulic pressure and power in a reaction hydraulic turbine. The system comprises means for introducing an additional, controlled, pulsating flow of water at the diffusor elbow, consisting, together or alternatively, of a) at least one moveable body inside the diffusor between a first, retracted position and a second, extended position; b) at least one auxiliary duct which is markedly parallel to the diffusor and connected to the said diffusor with its own inlet and outlet apertures the inlet opening being positioned at the end of the diffusor elbow controlled by a valve and the outlet opening being positioned adjacent to the piezometric basin; the said moveable body and valve being controlled in such a way as to make the volume of the diffusor vary to reduce the pulses of pressure in the diffusor to a minimum and to maintain the flow, the gap, the torque, and the power of the rotor of the turbine constant.
Document US 2004/037698 A1 describes an Exit Stay Apparatus for Francis and propeller hydraulic turbines. The purpose of the apparatus is to eliminate the loss of turbine efficiency and strong pulsations in draft tube caused by the axial circular vortex in all turbine operating regimes other than optimum without a noticeable decrease in maximum efficiency. The Exit Stay Apparatus has a crown and exit stay vanes secured to the crown. When installed in the turbine, the exit stay crown is located immediately after the runner crown, which is truncated at the bottom by a plane perpendicular to the central axis of the turbine. The exit stay crown together with the truncated runner crown forms water passages after the runner blade crown profile exit. The exit stay vanes are arranged in a circular array around the turbine axis, located after the runner blades, and attached at the periphery either to the draft tube cone or to an exit stay flange secured to the turbine discharge ring and to the draft tube cone.
Document WO 2008/060158 A2 relates to a hydraulic reaction turbine comprising a runner, a draft tube downstream of the runner, and injection means for introducing water into the draft tube in order to reduce pressure fluctuations therein, with one or more openings in the draft tube wall adapted to substantially evenly distribute injected water in the axial and/or circumferential direction of the draft tube.
Further examples of hydraulic turbine systems according to the prior art may be found in the following documents: JP 55060666 which discloses a method of miniaturising a turbine; JP 55051964 which discloses providing openings in the turbine to reduce water thrust; JP 06074138 which discloses a system which seeks to reduce friction loss; and U.S. Pat. No. 5,290,148 which is concerned with the thickness of a runner band and a crown, and rigidity in respect of structural matters.